The short answer is that one is not better than the other. Smiles are expected. No smiles are not. It just depends on what you want to say in the portrait. In a family group I think smiles are important otherwise people can look unfriendly or unhappy.

However, if I'm going for a little attitude in that portrait I'll definitely go for NO smiles. The non-smiling portraits in these examples won "hands down" with me and the parents. 

Non smiling doesn't always mean "having attitude" as in the examples. A neutral expression or slight smile without showing teeth can be the way to go.  When people don't smile big, they usually don't squint. If the eyes are the windows to the soul, then your more soulful portraits are not smiling!